1. Field of the Invention
The present invention is generally concerned with printing machines including an endless conveyor which carries a plurality of spaced object supports and at least one screenprinting station vertically aligned with the path of movement of the object supports and adapted to print the objects carried thereby in turn.
2. Description of the Prior Art
Screenprinting stations have many advantages which make them ideal for printing highly diverse articles, for example compact discs.
In particular, they have the advantage of a relatively high throughput, for example in the order of one compact disc every second, because the screen employed covers the entire surface to be printed and so prints it all in one operation.
They are therefore particularly suitable when the number of articles to be printed in series is relatively high, for example greater than one thousand.
However, screenprinting stations have the drawback of requiring adjustment of the position of the screen before printing is started, in order to adjust the position of the printing relative to the surface to be printed. Also, where appropriate, i.e. when a change of color is required, the squeegee and the bottom squeegee associated with the screen must be cleaned.
The corresponding start-up time can be as much as several minutes, for example.
Consequently, when a plurality of screenprinting stations are used to print the same article, with one screenprinting station for each color to be applied to the article, as is frequently the case, the overall time of operation of the printing machine is much greater than the actual printing time, for example three to four times greater than the printing time, which is to the detriment of productivity.
A general object of the present invention is an arrangement which addresses the problem of minimising the effects of the above drawback.
To be more precise, the present invention consists in a printing machine including an endless conveyor carrying a plurality of spaced object supports, at least one screenprinting station vertically aligned with the path of movement of the object supports and at least one digital inkjet printing station adapted to operate on demand.
Digital inkjet printing stations are well known and have the advantages that they operate directly, without any accessory such as a screen, plate, etc, and so save the cost of any such accessory, and that they can print more than one color without requiring specific adjustment for each color.
It is sufficient for them to include as many printing nozzles as there are colors to be printed and they are appropriately programed to control each printing nozzle in an appropriate manner.
However, operating on a point-by-point and line-by-line basis, digital inkjet printing stations have the drawback of a relatively low throughput, very much lower than that of a screenprinting station.
However, if the number of articles to be printed in series is relatively small, for example less than one thousand, the productivity of a printing machine employing a digital inkjet printing station becomes competitive with that of a printing machine using a screenprinting station, all the more so if more than one color is to be printed on the same article, as is usually the case.
By enabling the user of the printing machine of the invention to use a digital inkjet printing station, for example by substituting it for a screenprinting station, as and when required in accordance with current necessities, to be more precise in accordance with the number of articles to be printed, the invention has the advantage of maintaining acceptable productivity of the printing machine even when the number of articles to be printed is relatively small.
Furthermore, associating a digital inkjet printing station with a screenprinting station is even more advantageous in that inkjet printing requires a layer of absorbent material or varnish on the objects to be printed and a layer of absorbent material or varnish can readily be applied to the objects by a screenprinting station.
The features and advantages of the invention will emerge further from the following description given by way of example and with reference to the accompanying diagrammatic drawings.